The present invention relates to a glass substrate of an information recording medium used in a magnetic disk, a magneto-optic disk, or an optical disk, which are magnetic recording medium of information recording devices such as hard disks. The present invention also relates to a method for manufacturing such a glass substrate and a polishing pad used in the method.
Conventionally, to permit a glass substrate of an information recording medium (hereinafter also referred to as a glass substrate) to record high density information, the surface of the glass substrate needs to be as smooth as possible. Therefore, during manufacturing, a surface of a glass substrate is polished by supplying a polishing agent on the surface and rubbing the surface with a polishing pad so that the surface becomes smooth. For example, Japanese Laid-Open Patent Publication No. 2002-92867 discloses a glass substrate having an improved value of micro-waviness, which is one of the values representing the smoothness of the surface. In the publication, the micro-waviness of a surface of the glass substrate is improved by selecting the surface roughness of a polishing pad. This proposition utilizes a phenomenon that the value of the micro-waviness of a glass substrate depends on the surface roughness of a polishing pad.
However, since the polishing pad used in the above prior art includes foam, a number of pores are formed in the surface. Thus, the surface roughness of the polishing pad does not necessarily depend on the value of the micro-waviness of the glass substrate. That is, when measuring the surface roughness of a polishing pad with a probe meter, the pin of the probe meter enters pores formed in the surface of the polishing pad. Thus, the value of the surface roughness evaluated in the entire surface of the polishing pad reflects the depth of each pore. At this time, the influence of the depths of pores to the value of the surface roughness can be reduced by adjusting the cut-off value (λ). However, the pores have significantly varied depths, and it is practically impossible to measure the depths of all the pores. Therefore, it is extremely difficult to accurately measure the surface roughness by completely eliminating the influence of the depths of the pores. Thus, even if a measured surface roughness of a polishing pad has a desirable value, it is likely that the surface of the pad is rough. When such a polishing pad is used for polishing the surface of a glass substrate, the value of the micro-waviness on the surface is unlikely to have a desirable value.